The agency builds a case for putting Anwar al Awlaki, linked to the Ft. Hood shootings and Christmas bomb attempt, on its hit list. The complications involved are a window into a secretive process.

The list is scrutinized every six months, officials said, and in some cases names are removed if the intelligence on them has grown stale.

"If someone hadn't popped on the screen for over a year, or there was no intelligence linking him to known terrorists or plans, we'd take him off," the former official said.

The National Security Council oversees the program, which is based on a legal finding signed after the Sept. 11 attacks by then-President George W. Bush. But the CIA is given extensive latitude to execute the program, and generally does not need White House approval when adding names to the target list.

The only exception, officials said, would be when the name is a U.S. citizen's.

The CIA has at times considered adding Americans' names to the target list. None were ever approved, the officials said, not because their citizenship protected them but because they didn't meet the "continuing threat" threshold.

Adam Gadahn, a California native now believed to be hiding in Pakistan, has been indicted on charges of treason and providing support to Al Qaeda. But Gadahn, former officials said, has mainly served in a propaganda role.

Officials said that whether Awlaki is added to the list hinges more on intelligence agencies' understanding of his role than any concern about his status as a U.S. citizen.

"If you are a legitimate military target abroad -- a part of an enemy force -- the fact that you're a U.S. citizen doesn't change that," said Michael Edney, who served as deputy legal advisor to the National Security Council from 2007 until 2009.

Awlaki, 38, was known for delivering fiery sermons at mosques in San Diego and suburban Virginia before moving to Yemen in 2004. Because of his radical online postings, he has been portrayed as a catalyst or motivator in nearly a dozen terrorism cases in the U.S. and abroad.

But it was his involvement in the two recent cases that triggered new alarms. U.S. officials uncovered as many as 18 e-mails between Awlaki and Nidal Malik Hasan, a U.S. Army major accused of killing 13 people at Ft. Hood, Texas. Awlaki also has been tied to Umar Farouk Abdulmutallab, the Nigerian accused of attempting to detonate a bomb on a Detroit-bound flight.

"Awlaki's interested in operations outside of Yemen, and he's trying to recruit more extremists, including Westerners," said the U.S. counter-terrorism official. "His knowledge of Western culture and language makes him valuable to [the offshoot] Al Qaeda on the Arabian Peninsula.

"Taking him off the street," the official said of Awlaki, "would deal a blow to the group."

The CIA has carried out dozens of Predator strikes in Pakistan over the last year. The program is not foolproof, as drone strikes often kill multiple people even when the intended target escapes. The CIA has also made grievous mistakes in counter-terrorism operations, including capturing individuals misidentified as terrorism suspects. But the program remains valuable to U.S. officials.

President Obama alluded to the campaign in his State of the Union speech last week, saying that during his first year in office, "hundreds of Al Qaeda's fighters and affiliates, including many senior leaders, have been captured or killed -- far more than in 2008."

Many of those strikes were aimed at gatherings of militant groups or training complexes, current and former officials said. In such cases, the CIA is free to fire even if it does not have intelligence indicating the presence of anyone on its target list.

The CIA has carried out Predator attacks in Yemen since at least 2002, when a drone strike killed six suspected Al Qaeda operatives traveling in a vehicle across desert terrain.

The agency knew that one of the operatives was an American, Kamal Derwish, who was among those killed. Derwish was never on the CIA's target list, officials said, and the strike was aimed at a senior Al Qaeda operative, Qaed Sinan Harithi, accused of orchestrating the 2000 attack on the U.S. destroyer Cole.